Molding is a process by virtue of which a molded article can be formed from molding material (such as Polyethylene Teraphalate (PET), Polypropylene (PP) and the like) by using a molding system. Molding process (such as injection molding process) is used to produce various molded articles. One example of a molded article that can be formed, for example, from PET material is a preform that is capable of being subsequently blown into a beverage container, such as, a bottle and the like.
A typical injection molding system includes inter alia an injection unit, a clamp assembly and a mold assembly. The injection unit can be of a reciprocating screw type or of a two-stage type. Within the reciprocating screw type injection unit, raw material (such as PET pellets and the like) is fed through a hopper, which in turn feeds an inlet end of a plasticizing screw. The plasticizing screw is encapsulated in a barrel, which is heated by barrel heaters. Helical (or other) flights of the screw convey the raw material along an operational axis of the screw. Typically, a root diameter of the screw is progressively increased along the operational axis of the screw in a direction away from the inlet end.
As the raw material is being conveyed along the screw, it is sheared between the flights of the screw, the screw root and the inner surface of the barrel. The raw material is also subjected to some heat emitted by the barrel heaters and conducted through the barrel. As the shear level increases in line with the increasing root diameter, the raw material, gradually, turns into substantially homogenous melt. When a desired amount of the melt is accumulated in a space at discharge end of the screw (which is an opposite extreme of the screw vis-à-vis the inlet end), the screw is then forced forward (in a direction away from the inlet end thereof), forcing the desired amount of the melt into one or more molding cavities. Accordingly, it can be said that the screw performs two functions in the reciprocating type injection unit, namely (i) plasticizing of the raw material into a substantially homogeneous melt and (ii) injecting the substantially homogeneous melt into one or more molding cavities.
The two stage injection unit can be said to be substantially similar to the reciprocating type injection unit, other than the plasticizing and injection functions are separated. More specifically, an extruder screw, located in an extruder barrel, performs the plasticizing functions. Once a desired amount of the melt is accumulated, it is transferred into a melt accumulator, which is also sometimes referred in the industry as a “shooting pot”, the melt accumulator being equipped with an injection plunger, which performs the injection function.